Before carrying out electric discharge machining, a tool electrode is manufactured, having a shape that is the complement of a cavity to be formed, and having a size that is slightly reduced compared to the cavity. The tool electrode is positioned close enough to a workpiece to induce electric discharge. A microscopic gap between the tool electrode and the workpiece is filled with dielectric fluid.
If a power pulse is applied to the gap, after a delay time an electric discharge occurs and electrical current begins to flow across the gap. The delay time depends on the state of the gap and can not be predicted. If the application of the power pulse to the gap is stopped, insulation is restored into the gap. Thus, electric discharge is intermittently generated in the gap, and material is removed a little at a time from the workpiece. Pieces of the removed material are called chips, and are washed away from the gap by the dielectric fluid. The gap is restricted to a size that enables electric discharge to be efficiently induced. The size of the gap is a few pm to a few tens of μm. The tool electrode is moved a little at a time in a Z axis direction towards the workpiece, in response to removal of material from the workpiece.
A conventional sinker electric discharge machining apparatus includes a computerized numerical controller and an automatic programming device. The computerized numerical controller controls movement of the tool electrode and supply of power pulses etc. in accordance with a numerical controller (NC) program. Data representing the size and surface roughness of a cavity to be formed, material properties of the tool electrode and workpiece, etc. is input to the automatic programming device. Based on such data, the automatic programming device automatically creates an NC program for forming the cavity in a minimum amount of machining time. A process for forming the cavity is divided into a plurality of steps in which power pulse conditions and feed amount are set. The feed amount is a distance that the tool electrode is advanced in the Z axis direction towards the workpiece. If the tool electrode is moved by the set feed amount, the process proceeds to the next step in accordance with the NC program. At the next step, the setting of the power pulse conditions is switched. The power pulse conditions include, for example, on-time, off-time, current peak and voltage. In a first step, the feed amount is a distance from a reference position. Normally, the reference position is a position located away from an upper surface of the workpiece in the Z direction by a distance corresponding to a gap that will enable electric discharge to be induced. In the first step, a large power pulse is supplied to the gap, and the cavity is rough machined in the workpiece. If the large power pulse is supplied to the gap, the time required for machining is shortened, but the machined surface is undesirably rough. The feed amount in a second step is an additional distance from the tool electrode position at the time that the first step is completed. The feed amount in a third step is an additional distance from the tool electrode position at the time that the second step is completed. As the process advances to the second step and the third step of the process, the power pulse applied to the gap becomes smaller. In a final finishing step, extremely small power pulses are adopted for the purpose of making the surface roughness of the cavity fine. In the finishing step, ideally, the cavity is formed to the required dimensions.
In actual fact, however, the undesired removal of excess material from the workpiece constitutes a fatal mistake, which means that the cavity is often formed a few μm to over ten pm smaller than the required dimensions. The dimensions of the cavity are measured after formation of the cavity. Before measurement, the workpiece is washed and chips that are stuck to the workpiece are completely removed. In this way, an error between the measured dimensions and the required dimensions is obtained. The fact that surplus material having a dimension of a few μm to over ten μm remains may not be a major problem. However, in recent years, dimensional precision of a few pm or less has become necessary. In the manufacture of certain products, dimensional precision of within 1 μm has become necessary. A second process for removing surplus material without impairing the low surface roughness is called “additional machining”.
FIG. 5 shows a tool electrode 17 and a workpiece 18 before additional machining. Z represents the dimension of material to be removed, of a few μm to over ten μm. A represents the position of the tool electrode at the time when the finishing step is completed. A feed amount dZ equal to value Z is set in an NC program. In the additional machining a problem often arises where the NC program terminates without electric discharge being induced, even if the tool electrode 17 is moved from position A to position B. A gap size that at which electric discharge can be induced is mainly dependent on the magnitude of the power pulse. However, even under the same power pulse conditions, the gap size at which electric discharge can be induced differs due to the amount of chips present. Value H represents the gap size at the time where the finishing step is completed. Value G represents the gap size at the time where the additional machining starts. Electric discharge generated between the tool electrode and the workpiece is called primary discharge. Electric discharge arising through the microscopic chips is called secondary discharge. During machining, the chips float in the gap or attach to the workpiece. It is well known that if a greater amount of chips increases the occurrence of secondary discharge, the gap size is increased. At the time where additional machining starts, there are hardly any chips because of the washing at the time of measurement. Therefore, if an error e between the value H and the value G is larger than the value Z, electric discharge machining cannot be accomplished at all.
Japanese patent publication 2001-9639 discloses causing conductive powder to be attached to the surface of the workpiece, in order to prevent variation in the gap size.
Japanese patent publication 60-3933 discloses an electric discharge machine provided with a timer circuit capable of setting machining time. In a finishing step for removing very fine material, it is possible to terminate machining when the machining has reached a set time, instead of using feed amount.
The object of the present invention is to provide a sinker electric discharge machining method and device for accurately removing material having a very small size of a few μm to over ten μm using electric discharge.